The present invention relates generally to digital versatile discs and, more particularly, to a method for making digital versatile discs.
The latest generation of optical disc technology is the digital versatile disc (DVD), which is sometimes referred to as a digital video disc. A DVD has significantly more data capacity than a compact disc (CD). For example, the data capacity of DVD 5 is about 4.7 GB, which corresponds to the data capacity of about seven CDs. The data capacity of DVD 9 is about 8.5 GB, which corresponds to the data capacity of about thirteen CDs.
In the manufacturing of current DVDs, two 0.6 mm polycarbonate substrates are bonded together with an adhesive material. A number of techniques for bonding the substrates are currently in use. In each of these bonding techniques, coating of the substrates with an adhesive material may be achieved by capillary force.
One technique for bonding the substrates together is sometimes referred to as the drop technique. In this technique a bead of an adhesive material is first dispensed on a first substrate having a reflective layer formed thereover. A second substrate is then freely dropped on the first substrate from a height of about one-quarter of an inch. Finally, the first and second substrates are spun to spread the adhesive material therebetween to form an adhesive layer.
The drop technique is desirable because of its simplicity, but suffers from three key drawbacks that have heretofore restricted its use to DVD 5 fabrication. The first drawback is that air may be forced into or otherwise trapped in the adhesive material when the second substrate is dropped on the first substrate. This is problematic because air trapped in the adhesive material may cause air voids to be formed in the adhesive layer if the adhesive material does not fully wet the surfaces of the first and second substrates. The second drawback is that a thin adhesive layer, e.g., an adhesive layer having a thickness of about 40 .mu.m to about 70 .mu.m in accordance with DVD 9 specifications, cannot be formed within a short cycle time, e.g., about 4 seconds. The third drawback is that it is hard to control the uniformity of the adhesive layer. In spite of these drawbacks, the drop technique is suitable for use in DVD 5 process flows because the thickness and uniformity of the adhesive layer is not crucial in a DVD 5. The reason that the adhesive layer is not crucial in a DVD 5 is that all the recorded data is on the first substrate having the reflective layer formed thereover, and, consequently, the laser beam does not have to pass through the adhesive layer to read the recorded data. In contrast, in a DVD 9, which has recorded data on the second substrate that must be read by the laser beam after it has passed through the adhesive layer, the thickness and uniformity of the adhesive layer are crucial.
Another technique for bonding the substrates together is sometimes referred to as the gap fill technique. In this technique the first and second substrates are first brought close together so as to define a narrow gap therebetween. The outer edges of the substrates are then pulled apart by the application of force, e.g., with suction cups, and a hypodermic needle is inserted between the substrates. Once the hypodermic needle is in the proper position, a controlled amount of adhesive material is dispensed therefrom. After the hypodermic needle is withdrawn and the outer edges of the substrates are released, the first and second substrates are spun to spread the adhesive material therebetween to form an adhesive layer.
Attempts have been made to use the gap fill technique in DVD 9 process flows because it can form the requisite thin adhesive layer within a short cycle time, e.g., about 4 seconds. Problems have been experienced in these attempts, however, because the adhesive layer formed using the gap fill technique lacks uniformity. A further drawback of gap fill technique is that it is significantly more difficult to implement than the drop technique because it requires complex mechanical equipment, which is not only difficult to design, but also difficult to clean and maintain under manufacturing conditions.
In view of the foregoing, there is a need for a simple and reliable method of forming a relatively thin, uniform adhesive layer in a DVD that consistently achieves thorough wetting of the substrates with adhesive material.